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    SubjectRe: [PATCH] x86/boot/64: Clear CR4.PGE to disable global 1:1 mappings
    On Wed, 10 Apr 2024 at 14:58, Ingo Molnar <mingo@kernel.org> wrote:
    >
    >
    > * Ard Biesheuvel <ardb+git@google.com> wrote:
    >
    > > From: Ard Biesheuvel <ardb@kernel.org>
    > >
    > > The early 64-bit boot code must be entered with a 1:1 mapping of the
    > > bootable image, but it cannot operate without a 1:1 mapping of all the
    > > assets in memory that it accesses, and therefore, it creates such
    > > mappings for all known assets upfront, and additional ones on demand
    > > when a page fault happens on a memory address.
    > >
    > > These mappings are created with the global bit G set, as the flags used
    > > to create page table descriptors are based on __PAGE_KERNEL_LARGE_EXEC
    > > defined by the core kernel, even though the context where these mappings
    > > are used is very different.
    > >
    > > This means that the TLB maintenance carried out by the decompressor is
    > > not sufficient if it is entered with CR4.PGE enabled, which has been
    > > observed to happen with the stage0 bootloader of project Oak. While this
    > > is a dubious practice if no global mappings are being used to begin
    > > with, the decompressor is clearly at fault here for creating global
    > > mappings and not performing the appropriate TLB maintenance.
    > >
    > > Since commit
    > >
    > > f97b67a773cd84b ("x86/decompressor: Only call the trampoline when changing paging levels")
    > >
    > > CR4 is no longer modified by the decompressor if no change in the number
    > > of paging levels is needed. Before that, CR4 would always be set to a
    > > known value with PGE cleared.
    >
    > So if we do this for robustness & historical pre-f97b67a773cd84b
    > quirk-reliance's sake, I'd prefer if we loaded a known CR4 value again,
    > instead of just turning off the PGE bit.
    >
    > It's probably also a tiny bit faster, as no CR4 read has to be performed.
    >

    Fair enough. I'll go and change that.

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